Snap fit design in custom plastic parts is a common type of mechanical joining method. Although it looks simple, there is more to snap-fit connectors than meets the eye. For example, there are several types of snap-fit connectors, each suitable for different product design scenarios. In addition, there are several basic design tips that you must follow when designing snap-fit connectors. This article on plastic snap fit design guide will explain to you in detail.
What is Plastic Parts Snap Fit Design?
Snap fit design is a strategic approach to joining plastic components without external fasteners like screws, rivets or adhesives. This design technique assembly method used plastics’ flexibility and resilience to create interlocking features that allow components to “snap” into place with precision, creating a seamless and durable joint. Snap-fit design not only simplifies the assembly process but also enhances the overall structural integrity of the assembled parts.
Types of Plastic Snap Fit Design-Degree of Disassembly
Plastic snap connections mainly utilize the elastic deformation and recovery properties of plastic materials. Some snap joint have small engagements and cannot be disassembled due to limited deformation space. Some snap connections with large engagements have sufficient deformation space, making them easy to disassemble manually or with simple tools. So plastic snaps can be divided into easy-to-remove snaps, difficult-to-remove snaps, and non-removable snaps (dead snaps) according to the difficulty of disassembly.
Easy-to-remove snap connections: can be disassembled without tools.
Difficult-to-remove snap connections: require tools for disassembly.
Non-removable snap connections: can only be disassembled by destroying the components
Types of Plastic Snap Fit Design -Shape
Snap-fit designs are in various forms, each type is to specific requirements and applications. Here are three common types—Cantilever Snap Fit, Annular Snap Fit and Torsion Snap Fit.
Cantilever Snap Fit Design:
In this type, a protruding tab on one component securely engages with an undercut feature on another. The tab flexes during assembly, providing the necessary clearance for connection before snapping into place. Cantilever snap joint are ideal for lightweight applications where flexibility and ease of assembly are crucial. Common examples include consumer electronics housings and disposable packaging.
L-Shaped Joint
An L-shaped snap fit joint is a type of cantilever snap fit design commonly used in injection molded plastic parts. It features a projecting tab or hook on one component that locks into an L-shaped slot or recess on the mating component. L-shaped snap-fits are widely used in plastic product assemblies that require easy assembly and disassembly, such as control panels, electronic housings, and product casings.
U-Shape Joint
A U-shaped snap-fit joint is a type of Plastic snap fit design commonly used in injection molded plastic parts. It features a U-shaped cantilever beam on one component that deflects and snaps into an undercut or groove on the mating component. U-shaped snap-fits are commonly used in assemblies where a secure interlock is required, such as electronic housings, automotive interior trim, and consumer product casings. The U-shape design allows for easy assembly and disassembly while providing a strong snap-fit connection.
Annular Snap Fit Joint
An annular snap fit is a circular or ring-shaped protrusion on one component that snaps into a corresponding groove or channel on another. The circular geometry ensures a secure connection. This type is well-suited for applications requiring a 360-degree seal or where rotational stability is essential. Common examples like pen caps.
Torsion Snap Fit
Torsion snap fits rely on a twisting motion to engage and disengage components. The design incorporates spiraled or helical features, allowing for a controlled twisting action during assembly. Torsion snap fits are valuable in applications where a rotational connection is needed. They find use in caps for bottles, as well as in assemblies requiring a controlled rotational release.
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Plastic Snap Fit Design Calculation
Design calculations will vary depending on the type of design required. Symbols and formulas for effective calculations can help you do snap fit calculation better:
Symbols
y = permissible deflection
b = width at root
c = center of gravity (i.e., the distance between outer fiber and neutral fiber)
E as absolute value = percentage/100
E = permissible strain in the outer fiber at the root
l = length of arm
K = geometric factor
h = thickness at root
Es = secant modulus
P = permissible deflection force
Z = section modulus
Z = I c; where I = axial moment of inertia
Cantilever Snap Joint
A cantilever snap joint is subject to deflection, strain, and bending. Finding these three elements is essential to the design of a cantilever snap fit.
The maximum bending stress is given by
σmax = mc/I
where M is the maximum bending moment
C is the distance from the point of interest to the neutral axis
I is the moment of inertia
The maximum strain is given by
ε = M/IE
where E is Young’s modulus of the material
For a constant cross sectioned beam the deflection is given
by y=0.67 * ει²/h
where l is the length of the beam
h is the thickness at root
And the deflection force is given by
P =bh²/6 • Es ε/ι
where b is the width at root
Es is the secant modulus
Torsion Snap Joint
The shear modulus is given by
G=Es/2(1+ν)
where G is the shear modulus
Es is the secant modulus
ν is the Poisson’s ratio
The deflection force is given by
P • ι= γ GIp /r
where γ is the shear strain
Ip is the polar moment of inertia
W=P +tan(σ)/1-tan(σ) W=13.98 • 0.72+tan(30)/1-tan(30)
W=42.91 N
Annular Snap Joints
Transverse force is given by
P= y • d • Es • X
where y = undercut
d = diameter at the joint
Es = secant modulus
X = geometric factor
Mating force is given by
W=P • μ+tan(σ)/1-tan(σ)
where μ = coefficient of friction
α= lead angle
How to Design Snap Fit Joints?
Designing effective snap-fit joints requires careful consideration of various factors to ensure optimal performance and longevity.
Fillet the Base of the Cantilever:
Adding fillets to the base of the cantilever (protruding tab) reduces stress concentrations and enhances the flexibility of the snap-fit joint during assembly and disassembly.
Taper the Snap Fit:
Incorporating a taper in the snap-fit geometry facilitates easier engagement and disengagement of components. A tapered design reduces the required deflection force, making assembly more user-friendly.
Increase the Width of the Clip:
Increasing the width of the snap-fit clip enhances its strength and resistance to breakage, particularly in applications with higher loads. The snap-fit joints are to have a width of at least 5mm.
Consider Adding Lugs:
Lugs, small extensions, or tabs, can be added to the snap-fit design to provide additional stability and prevent unintentional disengagement.
Consider the Build Direction:
The orientation of the snap-fit features in relation to the build direction during manufacturing can influence the strength and performance of the joint.
3D Printing vs Injection Molding with Plastic Snap Fit Design
Choosing the right manufacturing process is crucial for snap-fit plastic parts. This quick comparison helps you understand the key differences between 3D printing and injection molding snap fit.
| Comparison | 3D Printing | Injection Molding |
|---|---|---|
| Best for | Prototyping, design validation | Mass production, final parts |
| Startup Cost | Low (no mold required) | High (requires mold) |
| Unit Cost | High | Low (cost-effective in large volume) |
| Design Flexibility | High (no draft or mold limitations) | Limited by mold structure |
| Strength & Durability | Moderate (short-term use) | High (suitable for repeated use) |
Common Snap-Fit Design Problems and Solutions
Snap Deflection is Not Enough:
In some cases, the deflection of the snap feature during assembly may not be sufficient, leading to difficulties in engaging or disengaging components.
Solutions: Ensure proper material selection and consider adjusting the geometry of the snap features. Conduct thorough prototyping and testing to validate the snap deflection under various conditions.
Stress Concentration:
High stress concentrations at specific points of the snap-fit joint can lead to premature failure or cracking.
Solution: Redistribute stress by optimizing the design, adjusting the geometry of the snap features, or incorporating fillets and radii. Finite Element Analysis (FEA) can aid in identifying and addressing stress concentration areas.
Stress Relaxation:
Problem: Over time, continuous stress on the snap-fit joint may lead to stress relaxation, reducing the joint’s effectiveness.
Solutions: Conduct long-term testing to understand the effects of stress relaxation. Choose materials with good fatigue resistance, and design the snap features with appropriate clearances to account for potential relaxation.
Occurrence of Creep:
Problem: Creep, the gradual deformation of a material under constant load, can compromise the integrity of the snap-fit joint over time.
Solutions: Select materials with low creep tendencies, conduct accelerated aging tests, and design the snap features with considerations for potential creep effects.
Repetitive or Fatigue Loading Failure:
Problem: Snap-fit joints subjected to repetitive loading may experience fatigue failure, leading to cracks or breakage.
Solutions: Consider fatigue-resistant materials, optimize the design for load distribution, and conduct fatigue testing to ensure the joint’s durability under repeated loading conditions.
Tolerance Issues:
Problem: Tight tolerances or variations in material properties may lead to difficulties in assembly or result in inconsistent performance.
Solutions: Design with generous tolerances, account for variations in material properties, and conduct regular quality control checks during production to ensure consistent snap-fit performance.
Advantages of Plastic Parts Snap-Fit Design
Efficiency and simplicity define the essence of snap-fit joints, offering a range of advantages that contribute to their widespread adoption in plastic component manufacturing.
Easy Assembly/Disassembly:
Snap-fit joints simplify assembly processes, reducing the time and effort required for putting components together. This ease of assembly extends to disassembly, making maintenance and repair tasks more straightforward.
No Extra Hardware:
One of the primary advantages of snap-fit joints is the elimination of additional hardware such as screws, bolts, or adhesives. This not only reduces material costs but also minimizes the complexity associated with managing and integrating multiple components.
Reliable and Long Service Life:
The interlocking features of snap-fit joints contribute to the overall durability of the assembled components. The absence of external fasteners means fewer points of potential failure, enhancing the reliability and extending the service life of the product.
Aesthetics:
Snap-fit joints often result in a cleaner and more aesthetically pleasing product appearance. Without the need for visible screws or fasteners, the external design can remain sleek and uncluttered, enhancing the overall visual appeal of the finished product.
Versatility in Design:
Snap-fit design allows for greater flexibility in product design. Engineers can explore creative and innovative designs without being constrained by the need for traditional fastening methods. This versatility extends to a wide range of industries, from consumer electronics to automotive applications.
Disadvantages of Plastic Parts Snap-Fit Design
While snap-fit joints offer several benefits, it’s important to learn the potential disadvantages of their use.
Complex Production Process:
Achieving precise and reliable snap fits can require complex tooling and molding processes. The design and production of molds for the interlocking features may add complexity to the overall manufacturing process.
Easy to Break:
Compared to some permanent joints, Snap-fit joints may have limitations in terms of load-bearing capacity. Depending on the application, excessive force or stress on the joint could lead to breakage, impacting the overall strength of the assembly.
Weaker than Permanent Joints:
In applications where extreme strength or rigidity is crucial, snap-fit joints may not always provide the same level of structural integrity as permanent joints such as welding or adhesive bonding.
Conclusion
Snap-fit joints are an important part of the product design industry, providing an efficient and convenient method to achieve durable mechanical joints. Snap-fit joints are an excellent choice for designing durable products that are also cost-effective. They offer manufacturers cost and performance utility. However, snap-fit design is a complex process. Therefore you need to learn the snap-fit joint types and follow the design principles. Zhongde is an experienced on-demand manufacturer in China. We have skilled engineers and mature manufacturing processes. Such as injection molding, compression molding, cast molding, etc. Welcome to upload the design files and get a manual quote.
